The use of modems is well known in data communication industry. Modems convert digital data to a modulated analog waveform that can be transmitted over a phone line and also receive from the phone line a modulated analog signal that is then converted to digital data. Initially designed to connect data terminals to host computers, modems are commonly seen today in many devices that require inexpensive, secure network connectivity. Examples include point-of-sale terminals, medical instruments, home satellite receivers, embedded control systems, remote diagnostics/maintenance, and web-enabled devices. Modems used in these devices are embedded modems. An embedded modem is different from a PC add-on card modem, which includes a standard bus interface, such as ISA bus or PCI bus, that can be plugged into a motherboard expansion slot. Most PC added-on card modems are soft modems, which leverage the huge computing power of CPU in PC to emulate the modem data communication protocol without the need of a special modem digital signal processor (DSP). An embedded modem is also different from a stand alone modem, which is connected to a system serial port through an external cable to perform serial communication functions. The physical size of an external modem is not a critical design constraint.
An example of embedded modem 100 is shown in FIG. 1. The embedded modem 100 contains a front-end analog circuitry, which includes a Direct Access Arrangement (DAA) 120, a set of discrete components 130, a Diode Bridge 140, and a telephone line interface 110 where TIP and RING 105 reside. A modem is connected to the TIP and RING of a telephone line. The TIP and RING 105 from the telephone line are input to a diode bridge 140 to rectify input polarity. The rectified signals are then input to a set of discrete components 130 and DAA 120 to monitor the line voltage and to control the on-hook/off-hook switching, AC termination, DC termination, and ringer impedance, etc. The front-end analog circuitry must meet global telephone line requirements with full compliance to FCC, CTR21, JATE, and other country-specific PTT (post, telegraph, and telephone) specifications. The interface 111 is a null interface simply to provide additional mechanical supports for the modem. The embedded modem 100 also contains a Digital Signal Processor (DSP) chip 160 to handle the modem communication protocol. The DSP interface to DAA and the interface to an external host are integrated in the DSP chip 160 to minimize component count. The interface 180-1, 180-2 to an external host can be configured to operate as a serial bus or a parallel bus.
A modem must be tested by a National Certification Body (NCB) to ensure the product is in conformance with the relevant standard before the modem is used. The relevant standard for Telecommunications (IT) and Information Technology (ITE) equipment is the IEC 60950. Besides the safety compliance to IEC 60950, each country may have requirements beyond IEC 60950.
Due to safety and homologation requirements, it is often advantageous to separate the modem design from the rest of system design and select a pre-homologated modem as an add-on component for the system. A separate modem component minimizes the homologation cost and task, speeds up the system design cycle, and avoids a defective modem causing an entire system to be set aside.
In a modem design, there is a portion that is isolated from the local ground. The components in this portion that are isolated from the local ground are called the TNV-3 (Telecommunications Network Voltage) circuits. The TNV-3 circuits are subject to ringing voltage and lightning surges, and are not considered to be touched by a user. Circuits powered by low-voltage DC supplies in which no hazardous voltages are generated are called SELV (Safety Extra Low Voltage) circuits. SELV circuits are safe to touch by user and include local ground. All components from the telephone line interface 110 to DAA device 120 are in the TNV-3 circuit 150. The modem DSP 160 is in the SELV circuit 190.
The area between the boundary of the TNV-3 circuit 150 and SELV circuit 190 is called the Isolation Barrier 199. It is a required insulation between the two circuits. For a modem to be compliant with global standards, under the worst-case conditions of ringing voltage and conductive dust pollutions, the required minimum insulation distance is 2.5 mm (0.1 inch) to prevent electrical arcing. The 2.5 mm minimum distance is applied from any printed circuit board (PCB) trace in the TNV-3 area 150 to SELV circuit 190 and from any PCB trace between TNV-3 150 and local ground 195. In FIG. 1, the high voltage capacitors C1, C4, C24, C25 155 are over the Isolation Barrier 199. The requirement of the Isolation Barrier 199 introduces constraints in the size of embedded modems as well as constraints in the placement of interface pins within the embedded modems. For example, both the Wintec SLM24xx DIP modem and the Conexant Socket Modem place interface pins at both sides of embedded modems to form a dual-in-line (DIP) package. FIG. 2 shows the placement of the interface pins on SLM24xx DIP modem 200. A DIP form factor is a common practice because most interface signals are on the SELV circuit side. But due to a high digital integration the SELV circuit occupies substantially less area than the TNV-3 circuit in an embedded modem. A solution to this design constraint is to place all digital interface signals around the SELV area, rather than to place only at one side of SELV area. For example, both DIP modem and Socket modem have 18 digital pins (including power/ground pins) connected to the SELV side, but only two interface signals (TIP and RING) are in the TNV-3 side. Thus, the high number of digital interface signals is divided and placed at both sides on these modems.
Another example is Insight's SLM2400i embedded modem, where the modem daughter card uses a dual-row header as the host interface connector at one end and a separate 4-pin strip header to connect TIP and RING signals to an external RJ11 phone jack at the opposite end of the modem.
The conventional approaches of placing interface signals at both sides of embedded modem in a dual-in-line form factor works fairly well, but there are some drawbacks. The conventional designs require two sets of pin headers in the modem assembly. The conventional designs also require two sets of sockets on the main-board to house the modem. All of these increase the modem manufacturing and integration cost. Another drawback is that a dual-in-line modem can be hard to insert into a socket on the main-board in case there is any misalignment or tilting between the socket and the header, etc.